This invention relates generally to fuel systems and more particularly to an improved starting system for a turbine engine.
One requirement of a turbine engine fuel system is that it be capable of generating a relatively large fuel pressure at low engine speed incident to start-up and engine acceleration. Known fuel control systems generally solve this problem by incorporating a variable displacement pump or by using an oversize fixed displacement pump to provide the required flow at low speeds. However, variable displacement pumps are complex, relatively unreliable, and more costly than fixed displacement pumps, and oversize fixed displacement pumps are inefficient over the normal operating range of the engine.
Therefore, there is a need for a fuel system that utilizes a fixed displacement pump to provide the necessary capacity and fuel metering accuracy required during turbine start-up and acceleration in accordance with a predetermined start-up and acceleration schedule.
Stated in another manner, typical fuel control systems for turbine engines incorporate a high speed engine gearbox mounted boost pump, filter, metering valve, pressure regulator, and control linkages or electronics. If the fuel tank is remote to the engine a tank boost pump is also used. Fuel flow is not possible without engine rotation of about 10% of rated speed. These systems are not consistent with low cost turbine engine system goals as the cost of these relatively complex components designed to withstand the harsh engine mounted environment are high, and their complexity results in relatively low reliability. Furthermore, any fuel flow desired to purge fuel lines of accumulated vapor, or air, is not possible prior to engine rotation. Such vapor, or air, if allowed to pass to the engine during starting, can lower starting reliability.